Detail the progression from the Ricardian, Specific Factors/HO, to Imperfect Competition models of trade. What changed along the way with regards to what these models could explain and why were these models were introduced? Be sure to describe important data features and trade facts explained/not explained. How would you empirically test for the validity of the Ricardian model of trade? What features of the data would have to be correlated? How would you empirically test for the validity that decreases in tariffs leads to higher exports between countries?